sheep grazing© Tim Scrivener

Sheep farmers are being warned to treat the predicted low risk of disease caused by liver fluke with caution as changing patterns of infection emerge.

Harriet Fuller, veterinary surgeon of Marches Veterinary Group, Leominster, said this doesn’t necessarily mean the risk of infection is low.

“Farmers must be aware that fluke may be present and causing production losses in their animals, although disease and deaths are not evident because the infestations are relatively light,” she said.

Speaking at the latest Eblex teleconference on Friday (19 December), she said this was because of the changing pattern of fluke infection.

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As the weather in early summer 2014 was drier than normal, all areas of England and Wales were predicted to have a low risk of disease that autumn, said Ms Fuller.

Because of the low risk, farmers may have delayed treatment at this time allowing eggs to be shed on pasture.

And as winter 2014 has been another mild one, infection rates could be widespread, but not so heavy, she added.

A key aim of any fluke control programme should therefore be minimising faecal egg excretion on to pasture, not just reducing risk of disease in the sheep.

How mild winters affect the life cycle

During winter 2013, most days were over 10C and at this temperature, any eggs excreted on to pasture from affected animals would’ve still hatched, explained Ms Fuller.

The miracidium that hatched out of the eggs had about four hours to find and infect a host mud snail, or they died. Mud snails prefer slowly moving water and slightly acidic conditions, but can also be found around standing water.

Those that did find a host snail were able to develop over the winter as temperatures did not drop significantly to below 5C. As a result, higher numbers of the next stage of the lifecycle, the cercaria, emerged earlier.

“Some farms saw early cases of acute fluke with deaths in sheep, while on other farms, fluke monitoring confirmed that infection had been picked up earlier in the year than normal.

“Where infections were light and disease was not observed, sheep may have been passing out fluke eggs for some time before any autumn fluke treatments were given,” added Ms Fuller.

How risk affects treatment choice

Because of the low autumn 2014 fluke risk, it was less important to use products containing triclabendazole on most farms at that time, said Ms Fuller, which also helped with increasing resistance towards this drug. Triclabendazole kills all stages, even immature fluke.

“Further treatments over the winter will depend on whether sheep have been grazing potential fluke habitats since the last treatment, the fluke history of the farm, environmental temperatures and when or if the sheep are to be housed,” she added.

For example, for those housing sheep in February, when temperatures are likely to be lower and new infection unlikely to be picked up, can use closantel or nitronyzil to kill the older fluke.

“Every farm is different and fluke treatments should be tailored to the individual situation and ongoing monitoring should be an integral part of control.”

Sheep should monitored for the appearance of fluke eggs in faeces to ensure treatment can be given whenever eggs are detected or to check the effectiveness of a treatment, explained Ms Fuller.

A new test, the coproantigen Elisa detects small fractions of liver fluke in faeces and may be more reliable in some situations as it can detect infection at an earlier stage.